A JS snippet which generates a random UUID for usage in Google Analytics' tracking code.

cookieless-analytics.js

const cyrb53 = function(str, seed = 0) {
   let h1 = 0xdeadbeef ^ seed,
       h2 = 0x41c6ce57 ^ seed;
   for (let i = 0, ch; i < str.length; i++) {
       ch = str.charCodeAt(i);
       h1 = Math.imul(h1 ^ ch, 2654435761);
       h2 = Math.imul(h2 ^ ch, 1597334677);
   }
   h1 = Math.imul(h1 ^ h1 >>> 16, 2246822507) ^ Math.imul(h2 ^ h2 >>> 13, 3266489909);
   h2 = Math.imul(h2 ^ h2 >>> 16, 2246822507) ^ Math.imul(h1 ^ h1 >>> 13, 3266489909);
   return 4294967296 * (2097151 & h2) + (h1 >>> 0);
};

let creationTime   = new Date().getTime();
let expiryTime     = creationTime + (30 * 24 * 3600 * 1000); // Change 30 to any number of days you want the UUID to be valid.
let clientIDSource = window.location.host + ";" + navigator.userAgent + ";" + navigator.language + ";" + creationTime;

// Only store client ID and its expiry time if it doesn't already exist and isn't expired yet.
if (window.localStorage) {
    clientIDhashed = localStorage.getItem('GA_CLIENT_ID_HASHED');
    clientIDexpiry = localStorage.getItem('GA_CLIENT_ID_EXPIRY');
    
    // Start a fresh session or update when expired. Otherwise, do nothing.
    if ((clientIDhashed === null && clientIDexpiry === null)
       || (clientIDhashed !== null && clientIDexpiry !== null && clientIDexpiry >= expiryTime)) {
        localStorage.setItem('GA_CLIENT_ID_HASHED', cyrb53(clientIDSource).toString(16));
        localStorage.setItem('GA_CLIENT_ID_EXPIRY', expiryTime);
    }
}